This invention relates generally to lateral position-sensitive devices (PSDs), and more particularly the invention relates to an improved PSD of tetralateral configuration.
Lateral effect position-sensitive devices (PSDs) are semiconductor structures including a p-doped layer and an n-doped layer separated by an intrinsic (i.e. undoped or lightly doped) semiconductor layer. Typically, the semiconductor material is silicon, but other semiconductor materials such as gallium arsenide have been used. Gold/beta-silicon carbide Schottky barriers have been examined, also.
The PSD relies on the lateral photoeffect first discovered by Schottky in 1930 and later exploited by Wallmark. See J. T. Wallmark, "A New Semiconductor Photo Cell Using Lateral Photo Effect," Proceedinqs IRE, Vol. 45 pp. 474-483, 1957; U.S. Pat. No. 3,028,500 for "Photoelectric Apparatus."
Several device geometries are known including the one-dimensional PSD in which two electrodes are positioned in spaced alignment on one doped surface, the duolateral structure in which a pair of electrodes are positioned in spaced alignment on each doped surface in 90.degree. orientation to give x and y outputs, and the tetralateral structure in which two pairs of electrodes are positioned in spaced alignment with 90.degree. orientation between pairs on only one doped layer to get x and y position outputs. An improved tetralateral structure, called a pin-cushion type, has curved outer boundaries of the p and n doped regions with point contacts at the four corners of one doped region. The pin-cushion PSD combines the best aspects of the duolateral PSD with those of the tetralateral PSD by putting all of the contacts on one side of the p-n junction and minimizing the interference between x and y outputs through boundary definition and electrode shape. Such a device is commercially available from Hamamatsu Corporation of Japan.
In operation, the p-n junction of the device is fully reverse-biased. A light spot incident on a doped surface generates an electric charge proportional to the light intensity at the incident position. This electric charge creates photo-currents which are detected by the electrodes. Movement of the light spot in the sensitive area of the doped layer between the electrodes changes the current on each of the electrodes whereby continuous position data can provide measurements of very small (submicron) spot movement.
The tetralateral PSD has several disadvantages relative to the duolateral PSD due to the placement of all electrodes on one surface. These include signal interference between the electrodes resulting in inherent nonlinearity and the division of the generated photocurrent into four parts so that resolution is roughly half that of the duolateral PSD. However, the tetralateral PSD has advantages when compared to the duolateral PSD including a faster response due to having both the x and y contacts on one surface, a much lower "dark current" or current inherent in a reverse-biased p-n junction even in a dark environment. Further, the tetralateral PSD has an easier bias application and a lower fabrication cost since only one side of the p-n junction requires close tolerance fabrication.
As noted above, the pin-cushion type of tetralateral PSD combines the best aspects of the duolateral PSD with those of the tetralateral PSD but still has limits in signal resolution, dark current, and output sensitivity.